Offshore well operations in deep water may employ a riser extending from subsea well equipment on the sea floor to a vessel or floating platform at the surface. During drilling, a drilling riser is connected to the subsea wellhead and extends to the drilling platform. During well production, production risers might extending from subsea well equipment, such as a subsea tree or manifold, to the surface platform.
Guides are employed between the riser and the opening in the vessel through which the riser passes. Typically a tubular conductor is mounted to and surrounds the riser. Bearing members, normally rollers, are mounted to the vessel and engage the conductor.
It is important to keep tension in these risers as the vessel rises and falls due to wave movement and/or currents. A tensioner assembly having hydro-pneumatic cylinder units is connected between the riser. As the vessel moves toward and away from the subsea wellhead, the cylinder units extend and retract to keep a generally uniform level of tension in the risers. Normally, the waves are not steep enough to cause the cylinder units to reach a maximum stroke position where the pistons bottom out on the cylinders. A possibility exists, however, that such waves could occur during extreme weather, such as hurricanes. If so, damage could occur to the cylinders.